In an automatic assembly line or the like in the automobile industry, for example, a workpiece is clamped in a clamping device for subjecting the workpiece to welding and other various kinds of processing. As such a clamping device, there are already-known devices as disclosed in patent documents 1, 2, and 3, for example. In each of these clamping devices, in general, a clamping arm is driven for rotation by a driving source and is moved to a predetermined clamping position which has been set in advance and then a large clamping force for clamping is generated by a toggle mechanism.
However, in the above prior-art clamping device, the position where the workpiece is clamped by the clamping arm has to be set in advance by adjusting the clamping device according to a size of the workpiece. Because the clamping position has been set according to the size of the workpiece, the clamping device needs to be stopped temporarily to reset the clamping position according to a size of a workpiece before clamping the workpiece of a different size. Moreover, when respective members forming a mechanism such as the toggle mechanism for transmitting a driving force from the driving source to the clamping arm wear as a result of repetition of operation, the clamping position is displaced and the workpiece cannot be clamped accurately. Therefore, the clamping device needs to be readjusted periodically to reset the clamping position.
As described above, in the conventionally-known clamping device, the above-described troublesome resetting operation of the clamping position is required so as to accurately clamp the workpiece in the clamping position by the clamping arm and an operation efficiency is decreased. They include the following:                (1) Patent Document 1 Japanese Patent Application Laid-open No. 2001-105332        (2) Patent Document 2 Japanese Patent Application Laid-open No. 2001-310225        (3) Patent Document 1 Japanese Patent Application Laid-open No. 2001-009741        